A conventional type of radial piston pump is comprised of a rotor, a plurality of pistons slidably inserted in respective cylinders formed in the rotor, and a cam ring.
The cam ring of a conventional type pump is fixed with the pump housing so that the top portion (radially outer end) of each piston slides along the radially inner surface of the cam ring. In order to reduce friction, that conventional type pump employs a shoe provided between the top of each piston and the radially inner surface of the cam ring. However, the shoe causes a problem in that it resists flow of fluid in the pump. Therefore, the rotor of that conventional type pump cannot operate at as high a rotational speed as may be desired.
Also in the prior art a rotatably supported cam ring is disclosed for use in a vane-type pump, as described in Japanese laid-open utility model application No. 57-180184. However, since the cam ring of that vane-type pump is used for forming a working chamber, the clearance between the cam ring and a housing must be minimal in order to prevent leaking of fluid. Moreover, since the cam ring is supported by the housing, the cam ring cannot change its position for varying the volume of the working chamber.